Polarized sunglasses have been used for many years to filter out reflections and in general to sharpen the viewed images of the surrounding scene. The polarization not only provides glare protection but also in some cases attenuates the overall light level such that the polarized sunglasses function not only as polarization lenses but also as light attenuating lenses.
U.S. Pat. No. 8,172,393 B2 by Tendler describes a lens that has an upper polarized portion and a lower unpolarised portion. The polarized upper portion of the lens provides glare protection and as the case may be increased light intensity protection. The unpolarised lower portion of the lens enables the wearer to view polarized displays, for example instruments in a car or aircraft cockpit.
This patent relates to a product patent with little information regarding how to manufacture this gradient polarization. In column 4, lines 51 to 55, of U.S. Pat. No. 8,172,393 B2, Tendler proposes that “Gradient polarization may be accomplished in a number of ways, one of which being the stretching of polarization layers to provide the gradient. Another way is to provide adjacent stripes of polarization material of different polarization densities.” These proposals are applicable; however, they may be expensive, in particular on a low number scale, or inadequate if to be adjusted to individual wearer's needs.
A polarizer is an optical filter that passes light of a specific polarization and blocks waves of other polarizations. It can convert a beam of light of undefined or mixed polarization into a beam with well-defined polarization, i.e., polarized light. The common types of polarizers are linear polarizers and circular polarizers. Polarizers are used in many optical techniques and instruments, and polarizing filters find applications not only in eyewear but also in photography and liquid crystal display technology. Polarizers can also be made for other types of electromagnetic waves besides light, such as radio waves, microwaves, and X-rays.
Linear polarizers can be divided into two general categories: absorptive polarizers, where the unwanted polarization states are absorbed by the device, and beam-splitting polarizers, where the unpolarized beam is split into two beams with opposite polarization state.
A plurality of absorptive polarizers are known. Certain crystals, due to the effects described by crystal optics, show dichroism, preferential absorption of light which is polarized in particular directions. They can therefore be used as linear polarizers. The best known crystal of this type is tourmaline. However, this crystal is seldom used as a polarizer, since the dichroic effect is strongly wavelength dependent and the crystal appears colored. Herapathite is also dichroic, and is not strongly colored, but is difficult to grow in large crystals.
Well known among polarizing films is in particular the Polaroid® polarizing filter. A Polaroid® polarizing filter functions similarly on an atomic scale to a wire-grid polarizer. It was originally made of microscopic herapathite crystals. Its current H-sheet form is made from polyvinyl alcohol (PVA) plastic with an iodine doping. Stretching of the sheet during manufacture causes the PVA chains to align in one particular direction. Valence electrons from the iodine dopant are able to move linearly along the polymer chains, but not transverse to them. So incident light polarized parallel to the chains is absorbed by the sheet; light polarized perpendicularly to the chains is transmitted.
In the meantime a plurality of polarizing films being derived from the original Polaroid® polarizing filter are known. Such polarizing films may roughly be divided into two types: iodine-type films with superior optical characteristics and dye-type films with superior heat-resistant characteristics.
The production of such a polarizing film is derived from the Polaroid® process. The polarizing film is either made by dyeing a substrate film of an in general polymeric material (mainly polyvinyl alcohol (PVA)) or by iodine being adsorbed onto its surface, then stretching and orientating the film. This gives the film polarization characteristics that allow only light with a certain oscillation direction to pass through it. Furthermore, in order to secure mechanical strength of the film, backing materials such as a triacetyl cellulose (TAC) film or a protective film are laminated to the polarizing film.
Examples for substrate films and dyes are for example disclosed in United States patent application publication 2008/0094549 A1.
The predominantly used material for substrate films is polyvinyl alcohol. Polyvinyl alcohol (PVOH, PVA, or PVA1) is a water-soluble synthetic polymer. It has the idealized formula [CH2CH(OH)]n. It is used in papermaking, textiles, and a variety of coatings. It is white (colorless) and odorless.
The polarizing film may be given various kinds of treatment. The film itself may also be required to have functions to control reflection, stain and light leakage. Some types of liquid crystal displays (LCDs), touch panels, or organic electroluminescence (EL) displays also require a product made by laminating a retardation film to the polarizing film.
The durability and practicality of a Polaroid® type polarizing filter makes it the most common type of polarizer in use, for example for sunglasses, photographic filters, and liquid crystal displays. It is also much cheaper than other types of polarizers.